I have my "Action after email is sent" field set to Display Text in English, and I've found a small bug: adding an apostrophe also adds a backslash.
If I enter the message "Thanks for your email! I'll try to get back to you as soon as I can." to the field, when I save it the message changes to "Thanks for your email! I\'ll try to get back to you as soon as I can."
Notice the extra backslash in "I\'ll." If you delete the erroneous backslash and save, it just keeps coming back.
The erroneous backslash also shows up on the site itself after filling out the contact form, so this isn't just a bug with the settings page.